Synthesis and characterization of closed-loop chemically recyclable epoxy thermosets using bio-derived imine-containing secondary amine hardeners

Epoxy thermosets are widely used because of their mechanical robustness, chemical resistance, and strong adhesion, yet their highly crosslinked network structures severely limit recyclability and pose major sustainability challenges. Herein, a recyclable epoxy thermoset platform based on bio-derived, imine-containing secondary amine hardeners is presented. The hardeners, synthesized from renewable aromatic aldehydes and constructed on a multifunctional cyclotriphosphazene core, enable the formation of well-defined epoxy networks while avoiding the structural heterogeneity typically associated with conventional primary amine–cured systems. A library of epoxy thermosets was prepared by curing the hardeners with epoxy monomers of different functionalities, enabling systematic structure–property comparisons. The resulting materials exhibit robust thermal and mechanical properties and undergo selective depolymerization under acidic conditions into a small number of well-defined depolymerization molecules. These molecules are directly reused to regenerate the original network, yielding recycled materials with properties similar to those of the virgin material and demonstrating closed-loop chemical recyclability at the molecular level. In addition, the epoxy networks display controlled, stimulus-responsive debonding behavior in coating applications, allowing efficient substrate recovery under acidic conditions while remaining stable in water. This work demonstrates an epoxy thermoset platform incorporating bio-derived hardeners that enables controlled depolymerization and reuse of the recovered components. • Liquid bio-derived secondary amine-based imine hardeners were synthesized. • Homogeneous epoxy networks were obtained and characterized. • Acidic depolymerization yielded only well-defined depolymerization products. • The epoxy network was regenerated with properties similar to the original material. • On-demand acidic debonding was demonstrated in a removable epoxy coating.